drm/i915: pwrite/pread do not require obj->base.filp, just pages

The idea behind relaxing the restriction for pread/pwrite was to handle
!obj->base.flip, i.e. non-shmemfs backed objects, which only requires
that the object provide struct pages.

v2: Remove excess (). Note enough editing after copy'n'paste.
v3: Use new i915_gem_object_has_struct_page()

Testcase: igt/prime_vgem/read
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466431552-17860-2-git-send-email-chris@chris-wilson.co.uk